Fiberglass sucker rod strings are becoming popular for use in producing oil wells since they may increase production over steel rod strings due to pump plunger over travel possibly because of higher elastic properties of the fiberglass reinforced plastic rods. Another desirable feature of the fiberglass sucker rod strings is that the overall weight of the string is reduced when compared to steel sucker rod strings. Still another advantage of the fiberglass rods are that they will not corrode.
In steel sucker rod strings, the steel sucker rods are screwed into threaded end fittings to enable the sucker rods tp be connected together in an end-to-end relationship. When fiberglass rods are used, it is not possible to threadably secure the fiberglass rods to the end fittings so an alternate connection means must be provided. One form of connecting a fiberglass rod to an end fitting is disclosed in my U.S. Pat. No. 4,297,787 which issued on Nov. 3, 1981. An end fitting specifically designed for fiberglass sucker rods is illustrated in U.S. Pat. No. 4,360,288. Still another means of securing a fiberglass rod to an end fitting is disclosed in U.S. Pat. No. 4,315,699.
In U.S. Pat. Nos. 4,297,787 and 4,315,699, an adhesive or epoxy resin material secures the fiberglass rod to the metal end fittings. It has been found that there can be a separation in the fiberglass rod from the end fitting due to failure of the epoxy bond within the compression fitting. The usual assembly procedure for bonding a fiberglass rod to a metal end fitting is as follows:
(a) Degrease the machined end fitting.
(b) Apply a mold release agent to prevent a bond between the epoxy and the metal of the end fitting.
(c) Bake the treated fittings to drive off solvents in the mold release agent to prevent an intermix of mold release and epoxy resin that can reduce or destroy the physical properties of the potting resin.
(d) Inject a two-part epoxy resin into the fitting and slowly place the fitting onto the end of an abraided fiberglass rod while the rod is being held at a 35.degree. to 45.degree. angle from the ground to prevent air-pocketing of the epoxy potting resin.
(e) Centralize the rod with a plastic ferrule by driving it in along and around the rod between the opening of the end fitting. The ferrule also serves as a retainer for the epoxy resin so that the long rods may be processed further in a horizontal position while preventing air pockets caused by "resin-loss".
(f) Thermally cure the epoxy potting resin by placing the assemblies in a curing oven which not only accomplishes a curing of the resin but also causes a desirable post-curing of that portion of the rod.
(g) Placing the rods in a suitable tensioning device to set the wedge or separating the bonded assembly from the metal interior surfaces and to start the compression forces perpendicular to the wedge angle.
Once the wedge has been set, the compression forces are started perpendicular to the angles formed with the steel end fitting. This process separates the bonded epoxy resin fiberglass rod portion approximately 1/16th of an inch to 3/32nd of an inch depending upon such things as the compressive strength of the rod, the distance between, or the clearance around (cushion) the fiberglass rod and the smallest diameter of the compression device, the compressive strength of the epoxy potting compound, the degree of wedge angle, the temperature, and the tensional force being applied.
The necessity for a multiplicity of angles is relative only with regard to improper degree of angle, choice of materials, or assembly technique. The use of more than one compression wedge angle or cavity becomes "fail-safe" only when the wedge angle cavity most removed from the tensile force is or has been improperly formed due to air pocketing or improper centralizing of the fiberglass rod with respect to the end fitting. The effect of improper centralizing of the rod is that unequal compression forces perpendicular to the wedge angle cause a cantilever point where a shear is applied to a rod with low shear resistance. A small percentage of the shear problems may be reduced by increasing the cushion distance. However, ultimate tensile failure does not occur to properly assembled parts with properly selected materials within the area of the compression angle most removed from the source of tension.
As long as proper lap shear strength is developed between the epoxy potting resin and the abraided fiberglass rod, and when the proper tension has been applied to set the wedge, the fiberglass rod will be held in compression sufficiently to make it useful to be joined end-to-end in a sucker rod string. The amount of compression to the fiberglass rod, after the initial force required to set the wedge is applied, will vary directly with the subsequent reciprocating tension, and most importantly, temperature. The epoxy wedge around the fiberglass rod will remain in compression as long as the formed assembly is in repose and as long as the working stresses are kept within the elastic working region of the fiberglass rod. When the fiberglass sucker rod strings are allowed to undergo compression, the wedges unseat or become loose and will work free. The reciprocating tension causes a movement and a wearing away of the epoxy resin which can unevenly alter the compression sufficient to cause an increased cantilever effect and resulting shear. The wear can also be sufficient to cause a release of the rod or "pull out". A wearing away of the potting resin can also increase the amount of compression upon the rod sufficient to crush the rod. The motion of the parts opposed to one another can also cause a delamination of a layer of fibers on the rod thereby causing a release of the potting resin and an eventual "pull out".
Therefore, it is a principal object of the invention to provide an improved end fitting for oil well sucker rods.
A further object of the invention is to provide an end fitting for oil well sucker rods which reduces or eliminates the likelihood of pull outs, shears or bond failures by preventing movement of the epoxy wedge system.
A further object of the invention is to provide an end fitting for oil well sucker rods including means for initially applying a longitudinal force on the end of the fiberglass rod within the end fitting.
Still another object of the invention is to provide an end fitting for an oil well sucker rod including an aperture which may be used to flood coolant therethrough while the interior configuration is being machined.
Still another object of the invention is to provide an end fitting for an oil well sucker rod which includes means for preventing movement of the wedge system within the end fitting.
Still another object of the invention is to provide an end fitting for an oil well sucker rod which substantially increases the compressive strength of the fiberglass rod.
Still another object of the invention is to provide an end fitting for an oil well sucker rod which is durable in use.
These and other objects will be apparent to those skilled in the art.